Phononic crystals are synthetic materials that are formed by arranging materials in a periodic array or lattice structure. The periodic structure of phononic crystals has profound effects on wave propagation throughout the crystals. Periodic variation of the acoustic properties of the material can introduce phononic bandgaps into the crystals, in which certain frequencies of phonons cannot propagate. It is possible to adjust the acoustic and thermal properties of the crystals by adjusting the phononic bandgaps.